Without limiting the scope of the present invention, its background is described with reference to separating a tubular string from a subsea well installation in an emergency situation, as an example.
In many deep water offshore well operations, tubular members such as risers, work strings, production strings and the like are connected between a floating surface vessel and a subsea well installation. Such offshore facilities include various safety systems that automatically prevent fluid communication between the well and the vessel in the event of emergency conditions such as when the vessel must drive away from the location of the subsea well installation. Such drive away conditions may occur as a result of a malfunction of one or more components of a positioning system of a dynamically positioned vessel, the breaking of a tensioned cable of a moored vessel, an effort to avoid bad weather or the like.
In certain installations, the safety systems may include a subsea safety tree on the lower end of the tubular string that may be positioned within the blowout preventer stack of a subsea wellhead. The subsea safety tree may include one or more shut-in valves that operate to automatically shut-in the well. In addition, the subsea safety tree may include a latch assembly that enables separation of the tubular string from the lower portion of the subsea safety tree, a retainer valve that prevents fluid discharge from the tubular string into the environment and a vent sleeve that provides for controlled venting of pressure trapped between the closed retainer valve and the closed shut-in valves of the subsea safety tree.
Conventionally, each of these components of the subsea safety tree, the retainer valve, the vent sleeve, the latch assembly and the shut-in valves, are controlled by fluid pressure in control lines which extend from a pressure source on the vessel to the subsea safety tree. In many installations, dedicated control lines between each of the components and the surface are used, including both supply lines and return lines. In addition, the actuation of each of these components is controlled by electrical switches, such as solenoid valves, that selectively prevent and allow hydraulic pressure to operate the various components. Accordingly, in an emergency situation wherein disconnection of the tubular string from the subsea well installation is required, the proper operation of each of these independent control systems is necessary to safely shut-in the well, contain fluid within the tubular string, bleed off pressure between the shut-in valves and the retainer valve and cause separation of the tubular string from the subsea well installation.
Therefore, a need has arisen for an apparatus and method for disconnecting a tubular string from a subsea well installation in an emergency situation that has improved reliability. A need has also arisen for such an apparatus and method that simplifies the control system required to execute an emergency disconnection of a tubular string from a subsea well installation. Further, a need has arisen for such an apparatus and method that reduces the time required to execute an emergency disconnection of a tubular string from a subsea well installation.